1. Statement of the Technical Field
The inventive arrangements relate generally to transmission line transformers, and more particularly for transmission line transformers that can be dynamically tuned.
2. Description of the Related Art
RF circuits commonly utilize transmission lines manufactured on specially designed substrate boards. In an RF circuit, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, inefficient power transfer, unnecessary heating of components, and other problems can result. A specific type of transmission line often used to match the impedances of different parts of the circuit is a transmission line transformer. Hence, the performance of transmission line transformers in printed circuits is often a critical design factor.
One common transmission line transformer is a quarter-wave transformer. As the name implies, a quarter-wave transformer typically has an electrical length precisely λ/4, where λ is the signal wavelength in the circuit. Notably, transformers that have other lengths also can be used, but impedance calculations are simplified when the length of a transformer is an integer multiple of λ/4. In particular, the characteristic impedance of a properly tuned quarter-wave transformer is given by the formula Z0√{square root over (Z1Z3)}, where Z0 is the desired characteristic impedance of the quarter-wave transformer, Z1 is the impedance of an input transmission line to be matched, and Z2 is the impedance of an output transmission line or load being matched to the input transmission line.
Printed transmission line transformers used in RF circuits can be formed in many different ways. One configuration known as microstrip, places the transmission line transformer on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the transmission line transformer is covered with a dielectric substrate material. In a third configuration known as stripline, the transmission line transformer is sandwiched within substrate between two electrically conductive (ground) planes.
Low permittivity printed circuit board materials are ordinarily selected for implementing RF circuit designs, including transmission line transformers. For example, polytetrafluoroethylene (PTFE) based composites such as RT/duroid® 6002 (permittivity of 2.94; loss tangent of 0.009) and RT/duroid® 588 0(permittivity of 2.2; loss tangent of 0.0007), both available from Rogers Microwave Products, Advanced Circuit Materials Division, 100 S. Roosevelt Ave, Chandler, Ariz. 85226, are common board material choices.
Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or εr) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (με)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the physical length of a transmission line transformer.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (H1/C1)}, where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures. Accordingly, the overall geometry of a transmission line transformer will be highly dependent on the permittivity and permeability of the dielectric substrate.
The electrical characteristics of transmission line transformers generally cannot be modified once formed on an RF circuit board. This is not a problem where only a fixed operational frequency and a fixed characteristic impedance are needed since the geometry of the transmission line transformer can be readily designed and fabricated to achieve the proper design parameters. When a variable characteristic impedance is needed or the transmission line transformer must operate over a range of frequencies, however, use of a transmission line transformer having fixed dimensions can be a problem.
In particular, a transmission line transformer length optimized for a first RF frequency may provide inferior performance when used at other frequencies due to variations in electrical length. Moreover, if the transmission line transformer characteristic impedance is optimized for particular source and load impedances, the transmission line transformer may provide an inadequate impedance match if the source or load impedances should vary.